The subject invention relates to the preparation of cyanate resin-based prepregs and films for use in advanced structural materials. More particularly, the subject invention relates to cyanate resin-based prepregs and films which maintain their tack. Such prepregs and films are useful as a component of structural composites and as high performance adhesives in aerospace, transportation, and other industries. 2. Description of the Related Art
The use of fiber-reinforced composite materials has increased markedly in recent years. Coupled with the increased use of such materials is the increased use of structural adhesives. Together, these two materials are replacing high-strength magnesium, aluminum, steel, and titanium in many applications while offering considerable savings in weight at the same time. The composites are generally prepared by stacking several layers, or plies, of fiber-reinforced prepregs together and curing under heat and pressure. The composites thus prepared may be adhesively joined to other composites or to metal structures, or may be joined by means of conventional fasteners such as screws, bolts, and rivets. Often, both conventional fasteners and adhesives are used concurrently.
Prepregs and adhesive films may be prepared by a variety of methods. Prepregs may be prepared by impregnating the fiber reinforcing substrate with resin from solution, from the melt, or by application under heat and pressure of one or more resin films. The resin films themselves may be cast from solution or prepared from the melt. Prepregs and films manufactured by these methods are generally prepared in the form of rolls, with a release film on one or both sides. Without such release films, considerable interlayer adhesion would occur, rendering the products either wholly useless, or at least very difficult to use in their intended applications.
The release film may be selected from a variety of materials. Polymeric materials have been used, for example polyalkylenes such as polyethylene and polypropylene, or substituted polyalkylenes such as polyvinyl and polyvinylidene chlorides and fluorides. Highly fluorinated polymers, such as polytetrafluoroethylene have been used when appropriate. Many of these polymer release films must still be coated with a release coating to provide the necessary release properties. Silicone coatings are most commonly used for this purpose.
Paper release films have certain advantages over other materials. In addition to being very economical, the paper release films generally possess greater stiffness than many of the polymeric films, which contributes to their ease of use. However, uncoated papers are usually, if not always, totally deficient in that they adhere too strongly to the film or prepreg, making their later removal difficult. They may also leave a residue of cellulosic fibers on the prepreg or adhesive surface after their removal. Further, in many cases, resin will adhere to the release film upon the latter's removal rendering the adhesive film or prepreg totally useless. Thus, coated release papers have proven to be necessary. The coating enables release during later processing while at the same time providing enough adherence to successfully prepare the film or prepreg. The release coatings which have been found to be most effective and at the same time economically attractive are the various silicone release coatings. Thus both polymeric and paper release films generally require a silicone release coating to be effective.
Resin systems which have been used in the past to prepare prepregs and adhesive films include the various phenolic resins, epoxy resins, polyimide and bismaleimide resins and the like. The use of cyanate-functional resins has recently been introduced for the manufacture of prepregs and adhesives. These cyanate resins have considerably enhanced physical properties over many of the conventional resin systems, including the bis-maleimides. At times, however, prepregs and films prepared from cyanate resin systems to which are attached silicone-coated release films would rapidly lose their tack, presenting a tack-free and sometimes powdery surface. Such a tack-free surface causes difficulties in handling the prepregs and films, and in achieving the requisite inter-ply fiber orientation. These difficulties are particularly pronounced during composite assembly. Furthermore, the composites prepared from tack-free prepregs can, in extreme cases, have lower interlaminar shear strength.